With a recent trend toward the value addition, size and weight reduction of portable appliances, there is a growing demand for high-energy secondary batteries. In such applications as electric vehicles, too, alkaline storage batteries are used as a mobile power source, and the development of high-energy secondary batteries has been hoped for. In answer to that market demand, research has been conducted on improvement of nickel-cadmium storage batteries and nickel-metal hydride storage batteries.
High-capacity electrodes have been developed and used as nickel positive electrodes for alkaline storage batteries for such purposes. They include sintered type electrodes prepared by filling a sintered nickel plaque with nickel hydroxide in a chemical process or electrochemical process and non-sintered type electrodes prepared by filling nickel hydroxide into high-porous foamed nickel or porous nickel fabrics.
The problem with those nickel electrodes is that the charge efficiency drops under a high-temperature atmosphere.
Attempts to overcome the problem have led to a new idea that cobalt is incorporated into nickel hydroxide to form a solid solution so as to shift the oxidation potential toward the "anodic" direction thereby improving the charge efficiency of nickel electrodes at high temperatures (Japanese Laid-Open Patent Publications Sho 59-68168 and Sho 63-152866).
Indeed, the charge efficiency of nickel electrodes at high temperatures can be raised that way. In this method, however, cobalt is incorporated uniformly in the nickel hydroxide for electrodes to form a solid solution, and has to be added in a large quantity in order to achieve a desired results. The problem with this is that a large amount of cobalt induces a reduction of nickel which is directly responsible for discharge capacity in the active material. In addition, the use of cobalt pushes up the production cost.
The object of the present invention is to provide a positive electrode active material for an alkaline storage battery with high charge efficiency at a high temperature without a substantial reduction in the percentage of nickel which determines the discharge capacity.
Another object of the present invention is to provide a positive electrode for an alkaline storage battery including the above-mentioned active material.